a. Field of the Invention
The invention relates to a probe for determining the "p" or "n" type conduction characteristic of a semiconductor.
b. Prior Art
In semiconductors, conduction of electricity is explained in terms of majority and minority carriers of electric charge. In n-type semiconductor materials, electrons are the majority carriers and holes, i.e. the spaces left by electrons, are the minority carriers. In p-type semiconductor materials the opposite is true: the holes are the majority carriers and the electrons are the minority carriers.
Semiconductor integrated circuits are often fabricated on a semiconductor wafer substrate, usually by methods similar to those described in U.S. Pat. No. 3,025,589 to J. A. Hoerni. It is necessary at the outset to know the p or n-type conduction characteristic of the wafer substrate.
Conduction type is determined by the kind of impurities, or dopants, added to pure silicon or germanium or the like to transform the pure material into a semiconductor. For example, if a trivalent impurity, such as boron, is added to a pure silicon crystal, a p-type semiconductor material results. On the other hand, if the chemical impurity, or dopant, has a valence of five, such as phosphorus, an excess electron would result with respect to each silicon atom. Phosphorus thus forms an n-type semiconductor material because electrons are the majority carrier.
Although manufacturers of semiconductor substrates, such as silicon and germanium wafers, specify p or n-type, sometimes the specification for a wafer is not known because of mix-ups in handling of the wafers, or other reasons. If integrated circuit manufacturers fail to test each wafer to be fabricated into a batch of hundreds of integrated circuits, one or more bad batches may occur. The time and expense for processing a batch is considerable in view of the relatively large amount of capital equipment required for wafer processing. Therefore, for quality control purposes, many manufacturers have a need for testing the p or n-type of each wafer to be processed.
Previously, manufacturers have used the "hot probe" technique to measure p or n-type. A hot probe consists of two electrodes, one of which is hot. The electrodes are placed in contact with a wafer, which is locally heated, increasing the mobility of majority carriers. Carriers flow away from the heated spot, causing the spot to acquire electrical charge opposite to the majority carriers. While the hot probe is widely used it is destructive because of thermal shock to the material tested.
Another test for p or n-type involves forming a contact diode with a wafer by means of a probe. The direction of current flow, either d.c. or a.c., through the diode indicates conduction type. The wafer usually sits on a conductive support to complete the circuit from the probe. A problem is that the metal support can readily contaminate large areas of the wafer.
An object of the present invention is to devise a non-destructive test method and apparatus for more quickly determining the p or n-type conduction characteristic of semiconductor materials, without contamination of the materials.